The present invention relates to methods, systems and apparatus for synchronizing digital signals, such as digital television (DTV) signals and other broadcast data signals, that are transmitted from a plurality of separated transmitting stations. It also allows the synchronization of the signal processing for transmission between a data source location and one or more transmitting locations.
When television or other transmitted signals cannot reach certain locations because of terrain blockage or because interference considerations require a lower than desirable transmitted power in a given direction, it is possible to fill in the signal at such locations through the use of additional transmitters called “boosters” or “gap fillers.” This approach is well known in the art. It is also possible to extend coverage and to achieve more uniform signal levels throughout a service area using “distributed transmission,” which serves similar purposes. Either approach results in a “single frequency network” of transmitters sharing the same channel. The term “booster” will be used herein to describe all such transmitters sharing a single channel.
When installing booster transmitters, advantage of the terrain often can be taken to keep the signals from the various transmitters isolated from one another to the fullest possible extent. Booster locations, antenna patterns and orientation, and power levels may be selected to maximize isolation of the signals and to place areas of overlap (i.e., areas of low carrier/interference (C/I) ratios, or high “internal” interference) where populations are minimal. Such measures are often inadequate, however, to avoid internal interference within the system.
When boosters are applied to analog signal coverage, including that of television signals, terrain blockage must be nearly complete since reception of signals from more than one transmitter will result in the appearance of echoes in the received signal, or ghosts in the received image. When digital signals are transmitted using boosters, the multiple signals arriving at a receiver still appear as a main signal and one or more echoes. In a digital receiver, however, it is possible to use adaptive equalizers, or other methods known in the art, to suppress the impact of the apparent echoes caused by the additional transmitters so as to permit extraction of the data despite the echoes. In some digital systems, moreover, it may be desirable intentionally to cause signal overlap since the receivers may be capable of combining the signal powers from several received signals, thereby recovering the signal at power levels below those that could be obtained from a single transmitter.
A necessary condition for making the signals from certain booster transmitters appear to receivers as echoes is that the signals transmitted from each transmitter used must be identical to those from the other transmitters in the network. In other words, in digital transmission, every sequence of bits on the input to the transmitters must produce an identical series of symbols for transmission from each transmitter output. This result can be achieved in one of two ways:    (1) A single modulator can be used and the modulated signal can be fed to each transmitter for relay; or    (2) A separate modulator can be used for each transmitter.
For a number of reasons, the use of a separate modulator at each transmitter will deliver higher performance from the system than would the relaying of signals from a single modulator. To create the effect of a transmitted main digital signal plus echoes, however, all of the modulators would have to be synchronized; that is, they would have to produce identical outputs from a given signal input.
A problem is that digital modulators often employ a number of processes that randomize the data that is fed to them. This randomization is done to enhance the transmission properties of the system. In some such systems, there currently is no way to cause all such modulators to adopt the same states at the same time. This is a necessary precondition for synchronizing transmitters when each has its own modulator.
In digital radio transmission and digital television systems using a single channel and the COFDM modulation technique, as are standard in certain regions of the world such as Japan and Europe, a special signal can be transmitted with the payload data to all modulators in the system to reset a number of circuit elements within the modulators to certain known states. This is possible because the data processing used is repetitive, passing through known states at particular times. The U.S. Federal Communications Commission (FCC), however, has adopted, for digital television in the United States, an 8-level vestigial sideband (8-VSB) modulation scheme with trellis coding (8T-VSB), documented by the Advanced Television Systems Committee (ATSC). The trellis coding method uses, in the coding process, memory that carries information across data structure boundaries, making it random relative to that data structure. The U.S. VSB system therefore has not been considered amenable to the processes used in single frequency networks, and modulators are not reset with this scheme. Similar characteristics exist in other modulation schemes wherein part of the data and/or signal processing carries information across data structure boundaries or in other ways has unsynchronized or non-repetitive processes.
Similarly, it may be desirable, in a system using signals having processes unsynchronized from the data structure, to separate some of the data or signal processing functions that normally take place in a modulator from other such functions, for example, at a source location and at one or more transmission locations. Yet it may be necessary to process the data at the source location in a way that requires knowledge of the states of some or all of the unsynchronized processing functions at the transmission location or locations. This might be useful, for example, to permit preprocessing of some or all of the signals to enhance their robustness or to permit combining of multiple signals at the source location in a way that takes advantage of some of the processing functions that normally would be performed at the transmission location or locations. Such separated processing would not normally be possible because of the unsynchronized processes usually performed at the transmission location or locations.